There is a three-pump system that is used as a hydraulic drive unit for a hydraulic excavator or other construction machine. The three-pump system includes three hydraulic pumps that are driven by an engine, and drives a plurality of hydraulic actuators through the use of hydraulic fluid discharged from the three hydraulic pumps. An example of the three-pump system is described in Patent Document 1. The three-pump system described in Patent Document 1 includes a first regulator and a second regulator. The first regulator controls the absorption torques of a first hydraulic pump and a second hydraulic pump by controlling the displacements of the first and second hydraulic pumps in accordance with the delivery pressures of the first and second hydraulic pumps. The second regulator controls the absorption torque of a third hydraulic pump by controlling the displacement of the third hydraulic pump in accordance with the delivery pressure of the third hydraulic pump. For the second regulator, spring means is employed to set a maximum absorption torque that is available to the third hydraulic pump. As regards the first regulator, a reference value for a maximum absorption torque available to the first and second hydraulic pumps, which is set by the spring means, is adjusted in accordance with the delivery pressure of the third hydraulic pump, which is introduced through a pressure reducing valve, to control the total absorption torque of the first, second, and third hydraulic pumps. The minimum delivery pressure within the delivery pressure range of the third hydraulic pump over which absorption torque control (or input torque limiting control) is exercised by the second regulator (the maximum delivery pressure within the delivery pressure range of the third hydraulic pump over which absorption torque control is not exercised by the second regulator) is set as a predefined pressure value for the pressure reducing value.
Patent Document 1: Japanese Patent JP-A-2002-242904